A common process sequence in manufacturing integrated circuits and other electronic devices is to deposit a dielectric layer over a semiconductor or metal region on a substrate, then etch a number of openings in the dielectric so that each opening exposes a contact area on the semiconductor or metal region, then deposit a metal plug in each opening so as to make electrical contact with the contact area. If the substrate is exposed to the ambient atmosphere or any other source of oxygen after the openings are formed, the surface of the semiconductor or metal layer exposed in each opening will become oxidized. This native oxide must be removed or "cleaned" before depositing the metal plug in order to achieve good electrical contact between the plug and the contact area.
One conventional method of cleaning native oxide from a silicon layer before depositing a tungsten plug is plasma cleaning with a mixture of argon and NF.sub.3, as described in commonly assigned U.S. Pat. No. 5,043,299 issued to Chang et al. on Aug 27, 1991. However, because this cleaning process includes argon, it is unsuitable for certain dielectrics or shallow semiconductor junctions which can be damaged by argon sputtering.
Another problem exists when the contact area is on a semiconductor region such as silicon. In order to make good electrical contact between a metal plug and the semiconductor region, it typically is necessary to deposit over the contact area a layer of refractory metal, and then anneal the workpiece so that atoms of the semiconductor material and the refractory metal interdiffuse to form a compound such as, when the semiconductor is silicon, a refractory metal silicide. The anneal temperatures required to achieve a low resistance electrical contact typically are high enough to produce undesirable diffusion of the refractory metal into the semiconductor layer or the dielectric.